Impedance matching pad for microwave heating and method of use



Nov. 26, 1957' GENTO I 2,814,298

H. F. AR IMPEDANCE MATCHING PAD FOR MICROWAVE HEAT NG AND METHOD OF USEFiled Nov. 18, 1953 2,814,298 Patented Nov. 26, 1957 EDAN CE NEATCHINGPAD FOR MICROWAVE IEATING AND METHOD OF USE Henry F. Argento, Glencoe,

Manufacturing Company, tion of Delaware Application November 18, 1953,Serial No. 392,970 Claims. (Cl. 128-404) 11]., assignor to RaytheonWaltham, Mass., a corpora- In heating a body, such as the human body,with microwaves, a large proportion of the energy is reflected from thesurface of the body and lost instead of being absorbed by the body.

Objects of the present invention are to avoid this loss and to causesubstantially all of the energy impinging on the body to be absorbed bythe body. Other objects are to provide for this purpose an impedancematching pad which is light and easy to handle, which will conform tothe contour of the body and which is durable in use.

According to the present invention the aforesaid reflection is avoidedby inserting in the path of the microwaves an impedance matching padwhose dielectric constant is the geometric mean of the dielectricconstants of the body and the air or other medium through which themicrowaves are transmitted to the body and whose thickness isapproximately equal to an odd number of quarter wave-lengths of themicrowaves in the pad, the pad being located a distance from the bodywhich is a multiple of one-half of the wave-length, including a zeromultiple, that is in contact with the body without any substantialspacing between the pad and the body. In this Way two reflections areproduced, one at each face of the pad. The two reflections beingsubstantially equal in amplitude and of opposite phase, theysubstantially cancel each other.

When the microwaves are transmitted through air, as they usually are,the aforesaid geometric mean is equal to the square root of thedielectric constant of the body. While the aforesaid pad may be spacedfrom the body one-half wave-length or other multiple of a halfwavelength, it is preferably placed in contact with the body, in whichcase it is preferably in the form of a flexible path which conforms tothe contour of the body. The preferred embodiment comprises a rubber padcontaining approximately 50% to 80% titanium dioxide.

For the purpose of illustration, a typical embodiment is shown in theaccompanying drawing, in which the figure is a diagrammatic view of anemitter E for irradiating a body B through the aforesaid pad P, theemitter comprising the usual antenna A, reflector R and coaxial inputterminal T for supplying current to the antenna.

While the composition of the matching impedance pad may vary widely,depending on the conditions of use and the wave-length, for diathermictreatment of the human body with a wave-length of 12.2 cm., the pad ispreferably made of butyl rubber loaded with titanium dioxide to increasethe dielectric constant of the pad. Inasmuch as the wave-length in thepad is equal to the Wave-length in air divided by the square root of thedielectric constant of the pad, increase in dielectric constant permitsa decrease in the thickness of the pad. The following are a few examplesfor a wave-length of 12.2

cm. in the region where the thickness of the pad approximate onecentimeter:

Titanium Example Dioxide, Density Dielectric Thickness percent Constantin Cm. by wt.

The composition of the pad is preferably made accord- The composition ispreferably compounded as follows. The butyl rubber is first milled fortwo or three minutes on a rubber mill with rolls preheated to about 60C. After this plasticizing operation the heating of the rolls isdiscontinued and the titanium dioxide is added gradually whilecontaining the milling from ten to twenty minutes depending on theamount of titanium dioxide. Then While the milling progresses, the zincoxide, stearic acid, Captax, Tuads and sulfur are admixed successively.After milling the finished composition for one minute, the rolls arecooled with water and the mixture is stripped from the rolls. Afterletting the material stand for at least twenty-four hours, it is cutinto discs of the desired size, the discs are stacked to the requiredthickness and inserted into a mold lubricated with magnesium stearate.After applying pressure of about eight hundred pounds per square inch,the resulting laminated pad is cured at about C. for an hour, afterwhich the finished pad may be removed from the mold.

Uneven heating of a body may be accomplished by varying either thethickness or the dielectric constant of the pad throughout its area,maximum heating occurring where the thickness is equal to an odd numberof quarter wave-lengths and the dielectric constant is equal to theaforesaid geometric mean and the amount of heating decreasing inproportion to departure from these optimum conditions. For example, if arestricted area, such as the eye of a patient, is to be protected whileheating the surrounding area, the portion of the pad covering thesurrounding area may have said optimum conditions and the thicknessand/or geometric mean of the portion of the pad corresponding to therestricted area may depart more or less from said optimum conditionsdepending upon the degree of shielding desired throughout the restrictedarea. Conversely if the microwaves from the emitter are not uniform inintensity throughout the area of the pad, the intensity may be madeuniform by varying the thickness and/ or geometric mean throughout thearea of the pad. For example, in a small emitter the intensity isusually maximum at the center and gradually decreases toward theperiphery; and to make the distribution uniform the pad may have saidoptimum conditions at the periphery and gradually depart therefromtoward the center. By making the distribution uniform maximum heatingmay be obtained throughout the area without danger of burning at anylocation within the area.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

I claim:

1. A therapeutic heating system comprising a microwave energy generatingmeans, means for directing the microwave energy to a body of livingtissue, an impedance matching pad placed in a first medium wherein thedielectric constant of said pad is the geometric mean of the dielectricconstants of said body and medium and whose thickness is approximatelyequal to an odd number of quarter wave-lengths of the microwaves in thepad for matching the impedance between said pad and said body anddistributing said energy over an area of said body, whereby when the padis placed in the path of the microwaves, at a distance from the bodywhich is a multiple of one-half the wave-length, loss by reflection isminimized and said microwave energy is free to pass therethrough.

2. A therapeutic heating system comprising a microwave energy generatingmeans, means for directing the microwave energy to a body of livingtissue, an impedance matching pad placed in a first medium wherein thedielectric constant of said pad is the square-root of that of the bodyand whose thickness is approximately equal to an odd number of quarterwave-lengths of the microwaves in the pad for matching the impedancebetween said pad and said body and distributing said energy over an areaof said body, whereby when the pad is placed in the path of themicrowaves, at a distance from the body which is a multiple of one-halfthe wave-length, loss by reflection is minimized and said microwaveenergy is free to pass therethrough.

3. A therapeutic heating system comprising a microwave energy generatingmeans, means for directing the microwave energy to a body of livingtissue, a rubber matching pad placed in a first medium wherein thedielectric constant of said pad is the geometric mean of the dielectricconstants of said body and medium and whose thickness is approximatelyequal to an odd number of quarter wave-lengths of the microwaves in thepad for matching the impedance between said pad and said body anddistributing said energy over an area of said body, said pad containingapproximately 50% to 80% titanium dioxide, whereby when the pad isplaced in the path of the microwaves, at a distance from the body whichis a multiple of one-half the wave-length, loss by reflection isminimized and said microwave energy is free to pass therethrough.

4. A therapeutic heating system comprising a microwave energy generatingmeans, means for directing the microwave energy to a body of livingtissue, a rubber matching pad placed in a first medium wherein thedielectric constant of said pad is the square-root of that of the bodyand whose thickness is approximately equal to an odd number of quarterwave-lengths of the microwaves in the pad for matching the impedancebetween said pad and said body and distributing said energy over an areaof said body, said pad containing approximately 50% to titanium dioxide,whereby when the pad is placed in the path of the microwaves, at adistance from the body which is a multiple of one-half the wavelength,loss by reflection is minimized and said microwave energy is free topass therethrough.

5. A therapeutic heating system comprising a microwave energy generatingmeans, means for directing the microwave energy to a body of livingtissue, an impedance matching pad placed in a first medium wherein thedielectric constant of said pad has a value which approximates thegeometric mean of the dielectric constants of said body and mediumthroughout at least a part of the area of the pad and whose thicknesshas a value which approximates an odd number of quarter wave-lengths ofthe microwaves in the pad throughout at least a part of the area of thepad for matching the impedance between said pad and said body anddistributing said energy over an area of said body, at least one of saidvalues varying throughout the area of the pad to control thedistribution of the microwaves which penetrate the body and saidmicrowave energy is free to passtherethrough.

References Cited in the file of this patent UNITED STATES PATENTS1,990,649 Ilberg Feb. 12, 1935 2,161,292 Hahnemann June 6, 19392,220,269 Patzold et a1. Nov. 5, 1940 2,242,886 Hirschland et al May 20,1941 2,304,540 Cassen Dec. 8, 1942 2,407,690 Southworth Sept. 17, 1.946

2,599,944 Salisbury June 10, 1952 FOREIGN PATENTS 155,953 Australia Aug.21, 1952 445,495 Great Britain Apr. 14, 1936 579,974 Great Britain Aug.22, 1946 894,421 Germany Oct. 26, 1953 OTHER REFERENCES Proceedings ofthe I. R. E., vol. 32, No. 4, April 1944, pp. 233-241 (Transmission-LineAnalogies of Plane Electromagnetic-Wave Reflections by Arthur Bronwell).

Application of Microwaves in Physical Medicine, a paper read at theConvention of the Institute of Radio Engineers at New York, N. Y., March4, 1952, 9 pp. (Copy in Division 55.)

